Pressure Relief Valve (PRV) | SKYbrary Aviation Safety

Description

Pressure relief valves are a critical component of an aircraft's fuel, hydraulic, and pneumatic systems. These valves serve one of three functions - safety relief, pressure regulation, or protection against thermal expansion.

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Safety relief valves

Safety relief valves are designed to be fast-acting components that protect systems with the potential for sudden pressure spikes. These valves may sit dormant for indeterminate amounts of time before needing to be used, so it&#;s crucial that they always work as intended.

Pressure regulating relief valves

Unlike safety relief valves, pressure regulating valves operate more frequently. These valves compensate for momentum forces that upset the normal force balance of typical direct acting valves.

Thermal relief valves

When traveling from the cold at an altitude of 30,000 feet to a runway in the desert, rapid temperature change can cause thermal expansion of fluids and potential damage within the system. Thermal relief valves are designed to relieve small volumes of fluid to protect against thermal expansion.

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Positive and negative pressure relief valves

Aircraft are built to withstand higher pressure on the inside of the aircraft compared to the outside ambient air pressure without damage. However, there is a limit to how much pressure they're designed to take. Thus the positive pressure relief valve opens if the pressure inside becomes greater than desired, so as to keep if from reaching the point where damage could occur. The positive pressure relief valve opens if the cabin pressure is too high in comparison to outside air pressure (pressure differential is higher than 8.85 psi / 0.61 bar). It is most likely to happen when there is a malfunction of the outflow valve (stays closed). 

On the other hand, aircraft are NOT designed nearly so robustly as far as pressures outside being greater than pressures inside. The negative pressure relief valve prevents the generation of sub-atmospheric pressures in the circuit as a result of a discrepancy between the fresh gas flow and the gas evacuation rate (pressure in the cabin from becoming lower than the ambient outside pressure). This situation is rare but can occur when an aircraft descends rapidly and the cabin altitude is still at 8,000 feet (normal condition) but the aircraft itself is lower than that. 

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